minimal form example

controllers.application\.js

import Controller from '@ember/controller';
import { action } from '@ember/object';

export default class ApplicationController extends Controller {
  @action submit(event) {
    event.preventDefault();
    const name = event.target.elements.namedItem('name').value;
		alert(`Your name is ${name}`);
    // do `fetch()` or something here!
  }
}

templates.application\.hbs

<form {{on "submit" this.submit}}>
  <label>
  	Your name:
    <input type='text' name='name'>
  </label>
  <button type='submit'>Save</button>
</form>

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}